In terms of lighting, watts/gallon is not a proper way to gauge lighting. It's best measured in pars, which for me translates to the intensity of light. From 13Watts/gal I'm guessing you have a 26W PC retrofit...and honestly, I also don't think the intensity will be enough for a maxima clam.
That being said, small tanks also have lots of fluctuations in parameters...temperature, salinity, and trace elements. Clams being big time Ca hogs, I can only imagine what problems you're gonna face when it comes to supplementing. I believe stability in a system is what will keep any live stock alive. Especially a clam like you describe, it sounds like a magnificent clam and I'd hate to not give it the best chance of survival. If you have some MH or even VHO or T5 in your other tanks, the clam will definitely do better in them IMHO.